CLISP open-http example - http

Question

I am trying to read a series of web pages with CLISP, if they exist, but I don't understand how open-http works to skip non existing web pages.
I have the following:
(dolist (word '(a b c))
(with-open-stream (stream (ext:open-http
(format nil
"https://en.wikipedia.org/wiki/~a.html"
word)
:if-does-not-exist nil))
(when stream
(print word))))
I want to simply skip a web-page if it doesn't exist, but CLISP seems to hang and returns an "Invalid argument" error.
Could anyone explain how the argument :if-does-not-exist works and/or provide examples of how to use open-http. Thanks!

Answer 1

It does work for me:
(with-open-stream (stream (ext:open-http
"http://stackoverflow.com/questions/234242424242"
:if-does-not-exist nil))
(format t "~&Stream: ~A~%" stream))
Output:
;; connecting to "http://stackoverflow.com/questions/234242424242"...connected...HTTP/1.1 404 Not Found
;; HTML source of Page not found
Stream: NIL
NIL
There is a delay to get the connection, but it works.
If the page does exist:
[7]> (with-open-stream (stream (ext:open-http
"http://stackoverflow.com/questions/36003343/clisp-open-http-example"
:if-does-not-exist nil))
(format t "~&Stream: ~A~%" stream))
;; connecting to "http://stackoverflow.com/questions/36003343/clisp-open-http-example"...connected...HTTP/1.1 200 OK
Stream: #<IO INPUT-BUFFERED SOCKET-STREAM CHARACTER stackoverflow.com:80>
NIL
With Wikipedia I couldn't make it work since Wikipedia.org re-directs it to HTTPS and EXT:OPEN-HTTP neither can handle HTTPS directly, nor it can handle redirects:
Here if HTTPS is used directly:
[10]> (with-open-stream (stream (ext:open-http
"https://en.wikipedia.org/wiki/Common_Lisp"
:if-does-not-exist nil))
(format t "~&Stream: ~A~%" stream))
*** - OPEN-HTTP: "https://en.wikipedia.org/wiki/Common_Lisp" is not an HTTP URL
The following restarts are available:
ABORT :R1 Abort main loop
Break 1 [11]> :r1
If "https" is replaced by "http", CLISP doesn't construct a proper address:
[12]> (with-open-stream (stream (ext:open-http
"http://en.wikipedia.org/wiki/Common_Lisp"
:if-does-not-exist nil))
(format t "~&Stream: ~A~%" stream))
;; connecting to "http://en.wikipedia.org/wiki/Common_Lisp"...connected...HTTP/1.1 301 TLS Redirect --> "https://en.wikipedia.org/wiki/Common_Lisp"
;; connecting to "http://en.wikipedia.orghttps://en.wikipedia.org/wiki/Common_Lisp"...
*** - PARSE-INTEGER: substring "" does not have integer syntax at position 0
The following restarts are available:
ABORT :R1 Abort main loop
Break 1 [13]>